DESCRIPTION (From the Applicant's Abstract): The development of specific neuronal connections requires the guided migration of growth cones. The mechanisms of how spatially segregated networks of actin and microtubules are coordinately assembled and dismantled during motility is a central issue in cell biology. This application aims to study the structure and function of Short Stop (Shot), an evolutionarily conserved molecule that likely coordinates actin and microtubule dynamics in growth cones. Shot is required in the Drosophila embryo for the extension of all sensory axons, the extension of all motor axons to target muscles, and the development of lateral branches in dendritic networks. Shot and related vertebrate genes encode a family of modular proteins that contain different N-terminal globular domains, central rod domains, and C-terminal domains. Despite this complexity, neuronal expression of a 5200 amino acid long isoform that contains an N-terminal actin binding domain, a central rod domain approximately 200 nm long, and a C-terminal microtubule binding domain fully rescues the axon defects in shot null mutant embryos. The C-terminal microtubule-binding domain stabilizes microtubules against depolymerization. Thus, Shot is likely to link actin to microtubules during axon and dendritic growth. These binding reactions are likely to be regulated by signaling pathways involved in motility, since Shot contains binding sites for a phospholipid second messenger near the actin binding domain and calcium near the microtubule binding domain. In addition, shot is allelic to kakapo, a gene implicated in integrin-mediated cell adhesion, raising the possibility that it may be involved in integrin-mediated signaling required for axon extension. This application seeks to investigate (1) whether Shot's interactions with actin and microtubules are required for axonal and dendritic development; (2) whether phospholipid, calcium, and integrin-mediated signaling regulate these interactions; and (3) whether other molecules that interact with Shot are required for neuronal morphogenesis. The ability to manipulate the structure of a functional Shot-GFP fusion in the fly embryo and in culture, together with genetic and developmental markers, provides powerful means for addressing these questions. Abnormal neuronal connectivity, cell motility, and adhesion are hallmarks of many diseases including epilepsy and cancer, and efforts to treat these conditions will likely benefit from a deeper understanding of Shot's role in morphogenesis.